(1) Field of the Invention
This invention relates to an antenna system for sharing of operation by a number of operators, and, more particularly but not exclusively, to such a system for use in cellular mobile radio systems. The antenna system of the invention is intended for use in many phased array applications such as radar and telecommunications, but finds particular application in cellular mobile radio networks, commonly referred to as mobile telephone networks. Such networks include the second generation (2G) mobile telephone networks such as the GSM, CDMA (IS95), D-AMPS (IS136) and PCS systems, and third generation (3G) mobile telephone networks such as the Universal Mobile Telephone System (UMTS), and other cellular radio systems.
(2) Description of the Art
Operators of conventional cellular radio networks generally employ their own base stations each of which is connected to one or more antennas. Because the numbers of cellular radio networks and operators are increasing world-wide, both the number of antenna sites and the number of antennas per site is increasing. Legal authorities responsible for planning or zoning arrangements are concerned to minimise visual impact of antennas on the environment: they are increasingly imposing restrictions such as limits on numbers of antenna sites and obtrusiveness of antenna structures. Antenna sharing has potential for alleviating the problem of limiting site and antenna numbers. However, it introduces problems of RF signal power losses in signal combining, and reduced flexibility as regards signal polarisation options.
RF signal power losses in signal combining occur as follows: in transmit mode, it is important to avoid mixing of different transmit frequencies, because this gives rise to unwanted intermodulation products. To avoid this, it is known to use 3 dB combiners to combine signals while providing isolation between pre-combined signals. U.S. Pat. Nos. 5,229,729 and 5,584,058 disclose combining signals using one or more 3 dB combiners each with one port terminated in a resistive load. Resistive loads have the function of dissipating RF power which cannot usefully be employed, and must be disposed of to avoid undesirable effects on required signals; each 3 dB combiner consequently introduces a 50% power loss.
An antenna system for shared operation by multiple operators of base stations is disclosed by European patent no. EP 0 566 603. This patent describes multiple base stations of different types (GSM, ETACS, TACS) connected to respective band-pass transmit filters and thence to a common transmit antenna. Signal polarisation, isolation and combining power loss are not addressed.
In order to improve transmission performance, it is known to use diversity, i.e. to receive and/or transmit two or more diverse signals. Diverse signals are processed either individually or in combination. There are three common types of diversity, a) frequency diversity, b) spatial diversity and c) polarisation diversity. In transmit mode, a mobile cellular radio handset has a single antenna which transmits a carrier wave with a single polarisation. A base station uses a dual polarisation antenna with one antenna element (or set of elements) having a +45 degree polarisation and the other −45 degree polarisation. A signal from the handset therefore gives rise to two signals at the base station antenna. The base station processes both received signals and obtains an improved signal. This approach combats changes in the polarisation of radio signals due to different orientations of the handset's antenna and reflection at buildings etc., which cause signals to be received at a base station antenna with multiple polarisations.
Published International Application No. WO 02/0082581 discloses a technique for combining a set of signals in which pairs of signals which are adjacent in frequency have contiguous frequencies. This technique both provides pre-combined signal isolation and avoids incurring signal power loss in 3 dB combiners mentioned above. Treating signals in the set as being numbered sequentially in order of frequency, the WO 02/0082581 technique groups the signals into odd and even numbered sub-groups of non-contiguous frequencies in each case. The odd numbered sub-group is connected to antenna elements of one polarisation in an antenna system, and the even numbered sub-group is connected to antenna elements of an orthogonal polarisation in this antenna system. Signals in the two sub-groups become combined in transmit mode when radiated from the antenna. Combining in this way is referred to as air combining, because transmit signals are not combined within the antenna system but upon radiation from it into air.
The technique disclosed in WO 02/0082581 is appropriate for slant polarisation antennas, such as +45 degrees and −45 degrees relative to the vertical: it can however be desirable to have capability for implementing both vertical and circular polarisation which can improve communications performance. The WO 02/0082581 technique is also appropriate for transmit and received signals having the same polarisation. Slant polarisation is known to improve communications performance in the case of received signals, but it is not an optimum polarisation for transmit signals. This is because a receive antenna in a mobile telephone handset may become oriented orthogonally to a slant polarised transmit signal or nearly or effectively so (having regard to signal reflections), which results in partial or even complete loss of received signal at the handset.